Conventionally, a known thermal-type airflow measuring device is employed to measure an air flow rate according to heat transfer from a heating element to air. Such a known thermal-type airflow measuring device is employed for measuring a flow rate of air, which is, for example, drawn into an internal combustion engine. In view of enhancing response and measurement accuracy, one example of a thermal-type airflow measuring device employs a semiconductor substrate equipped with a sensing element such as a heating element.
One example of a generally-known airflow measuring device includes a sensing element, a circuit board, and a support member. The sensing element is, for example, a heating element located in a predetermined passage and configured to change its electric input-and-output state according to a flow rate of air in the passage. The circuit board is a rectangular flat component having a hollow space depressed from its rear side toward its front side. A thin film is equipped to the circuit board to partition the front side of the hollow space. The sensing element is equipped on the surface of the thin film. The support member has a predetermined recess accommodating the circuit board. The support member supports the circuit board, such that the front side of the circuit board is exposed in the passage, and the longitudinal direction of the circuit board is substantially perpendicular to the flow direction of air passing on the front side of the circuit board.
The circuit board accommodated in the recess needs to be affixed to the support member. Therefore, adhesive is applied to, for example, the rear surface of the circuit board and bonded to the bottom surface of the recess with the adhesive. In such a configuration where the circuit board is bonded with adhesion, distortion may occur in the circuit board due to the difference in linear expansion between the circuit board and the support member to result in change in the characteristic of the sensing element. In consideration of this, for example, adhesive is applied only to the rear surface of the circuit board on one side in the longitudinal direction, and the circuit board is bonded to the bottom surface of the recess. Thus, the circuit board in the recess is supported by the bottom surface via the rear surface only on one side to form a cantilever configuration.
In such a cantilever configuration, the rear surface of the circuit board and the bottom surface of the recess form a gap therebetween. Therefore, air passing through the passage partially flows along the front side of the circuit board and partially flows into the gap on the rear side of the circuit board. Thus, underflow of air is caused in the gap one the rear side of the circuit board. When the underflow causes turbulence, measurement accuracy of the airflow measuring device decreases. In consideration of this, it is conceivable to employ a configuration to stabilize the underflow and/or to restrict air from passing on the rear side of the thin film.
For example, in an airflow measuring device according to a patent document 1, a groove is formed in the bottom surface of the recess at the position opposed to the periphery of the circuit board. In addition, the groove is partially enlarged outward beyond the periphery. The configuration of the patent document 1 is supposed to restrict the underflow from flowing on the rear side of the thin film and to reduce adverse effect caused by the underflow and exerted to the sensing element on the surface of the thin film.
It is noted that, in the configuration of the patent document 1, the configuration of the groove is complicated to restrict sufficiently the underflow from flowing on the rear side of the thin film. Specifically, the support member needs to be divided into multiple members and needs to be assembled into a singular component to result in increase in cost and increase in its manufacturing process. Furthermore, in the configuration of the patent document 1, characteristics of the assembled support member may have a large variation thereamong due to, for example, dimensional tolerance of the components of the support member. In addition, in the configuration of the patent document 1, the groove is formed in the enlarged area not to be entirely covered with the circuit board, and therefore, dust may adhere partially to the enlarged area and may stack in the groove. In this case, such dust stuck in the groove may cause fluctuation in both the underflow and airflow on the front side of the circuit board to impair the measurement accuracy.
In an airflow measuring device according to a patent document 2, adhesive is further applied to the lateral surface and the rear surface of a circuit board on the upstream side, in addition to the rear surface of the circuit board on one side. The configuration of the patent document 2 is supposed to restrict the underflow from passing on the rear side of a thin film thereby to reduce adverse effect caused by the underflow and exerted on the surface of a thin film.
Nevertheless, in the configuration of the patent document 2, the enlarged application region with adhesive may cause distortion in the circuit board to result in fluctuation of the characteristics of the sensing element. In addition, characteristics of the sensing element may have a large variation thereamong due to variation in the enlarged application region causing large variation in distortion in the circuit board.
In an airflow measuring device according to a patent document 3, the bottom surface of a recess has a groove located on one side of a thin film to extend from the upstream to the downstream. The configuration of the patent document 3 is supposed to restrict the underflow from passing on the rear side of the thin film thereby to reduce adverse effect caused by the underflow and exerted on the surface of the thin film.
Nevertheless, similarly to the configuration of the patent document 1, the groove is formed in the enlarged area not to be entirely covered with the circuit board, and therefore, dust may adhere partially to the enlarged area and may stack in the groove. In this case, such dust stuck in the groove may cause fluctuation in both the underflow and airflow on the front side of the circuit board to impair the measurement accuracy.
A patent document 4 discloses an airflow measuring device with a configuration supposed to reduce its manufacturing cost, to reduce fluctuation in characteristic, to reduce variation among products, and to reduce accumulation of dust. Specifically, in the airflow measuring device according to the patent document 4, the bottom surface of a recess has a groove, which overlaps with a thin film and is enlarged toward the upstream thereby to reduce turbulence of the underflow and to stabilize an output signal from a sensing portion with the overlapping configuration between the groove and the thin film.
It is noted that, an airflow measuring device is strongly demanded to have an extended detection range at the high flow rate side. It is conceivable that, as the flow rate increases, further turbulence may cause in the underflow. Therefore, further improvement is desired in order to enable extension of the detection range at the high flow rate side.
(Patent document 1)
U.S. Pat. No. 5,723,784 corresponding to Japanese patent No. 3967402
(Patent document 2)
Publication of unexamined US patent application No. 20100313651 corresponding to Publication of unexamined Japanese patent application No. 2010-286393
(Patent document 3)
Publication of unexamined US patent application No. 20080053215 corresponding to Publication of unexamined Japanese patent application No. 2008-058131
(Patent document 4)
Publication of unexamined Japanese patent application No. 2010-281809